This invention relates to an actuator, and in particular to an actuator suitable for use in aerospace applications for use in driving a moveable component between stowed and deployed positions. The invention is particularly applicable to an actuator of the type in which a lock arrangement is provided to resist extension thereof when this is not desired, for example an actuator suitable for use in controlling the deployment of thrust reverser cowls or the like. However, it will be appreciated that the actuator may be used in other applications.
In some applications, as mentioned above, it is important to be able to lock an actuator against movement in order to guard against movement of an associated moveable component other than when required. For example it is important to be able to lock the actuators associated with parts of a thrust reverser system against movement, the lock arrangements used to guard against movement of the moveable component being releaseable under the control of the thrust reverser control system or part of an associated engine control unit when deployment of the moveable component is required. The manner in which the actuator is locked against movement is preferably such that unwanted extension of the actuator either due to the application of external loadings or by the application of drive power to the actuator at times other than when desired is prevented.
One type of actuator commonly used in this type of application comprises a rotatable screw shaft with which a nut cooperates through the intermediary of a ball or roller screw coupling, the nut being held against rotation, for example by virtue of the manner in which it is secured to the moveable component. In use a motor, for example an electrically powered motor, is used to drive the screw shaft for rotation. The rotation of the screw shaft results in axial displacement of the nut, driving the moveable component for movement. The direction of movement of the moveable component is dependent upon the direction of rotation of the screw shaft.
In order to guard against deployment of the moveable component other than when desired it is known to provide a lock arrangement including a lock sleeve which can directly engage the nut. An actuator arrangement, for example in the form of a solenoid actuator, is provided to move the lock sleeve. Arrangements of this type are described in, for example, EP 2048414.
In such arrangements, the nut and lock sleeve are often exposed to environmental conditions which can result in contamination and/or icing that may impede the movement of the lock sleeve, and thereby interfere with the operation of the actuator. In order to overcome icing problems, it may be required to use a solenoid actuator arrangement of greater power than would otherwise be required to ensure that the lock sleeve can still be driven for movement, even when ice formation is impeding such movement. As solenoid actuators of increased power are typically also of increased weight, it will be appreciated that such a solution carries a weight penalty and so is undesirable.